Water and aqueous liquid absorbing hydrogel-forming polymers have been prepared from unsaturated monomers which are water-soluble acid group containing monomers and one or several cross-linking agents having at least two olefinically unsaturated groups in the molecule. Unsaturated water-soluble acid group containing monomers include acrylic acid or derivatives thereof which may be partially neutralized with alkaline. Polymerization may be performed under aqueous conditions or by the process of inverse suspension- or emulsion-polymerization as described in U.S. Pat. Nos. 4,286,082 and 4,340,706 or German Patent Nos. DE-PS 27 06 135 and DE-PS 28 40 010.
Other methods of producing hydrogels from polymers have been described by the production of graft-polymers using modified starch and cellulose (DE-OS 26 12 846) or polyvinylalcohol (DE-PS 40 20 780). The polymer-gel-particles or the powdery resin may be cross-linked on the surface, as described in German Patent No. DE-PS 40 20 780 C1.
Another method of producing hydrogel-forming polymers includes polymerizing partially neutralized acid group-containing monomers which were treated with alkaline prior to the polymerization as discussed in EP 205 674 A1. After the monomers and cross-linking agents react to form a cross-linked polymer material, some of the acid groups are partially neutralized during extrusion. The polymerization occurs within a temperature of 0 EC to 100° C., and preferably within a temperature of 5 EC to 40° C.
The above-discussed polymers as produced by different methods are known as “superabsorbers.” The absorption characteristics of superabsorbers make them useful in hygienic and sanitary applications. The superabsorbers must be skin-neutral and completely absorb the secretion rapidly. Therefore, 50 to 80 mol % of the monomers used to form the polymer are acid-group containing monomers which have been neutralized with a salt-forming cation. Greater absorption can be achieved by heat treatment of dried and shredded polymer-particles in presence of a cross-linking agent (DE-PS 40 20 780 C1).
Hydrogel-forming polymers may be used for water-storing, however, botanical use hydrogels have different requirements than the hydrogels described above. For example, plants grow best when the whole soil structure in combination with superabsorbers has been considered. Characteristics such as climate and capillarity of soil are just as important as water-capacity. Superabsorber polymers capable of releasing minerals and nutrients were made, however, the polymers were very sensitive to basic earth and UV-light. Consequently, alkali-silicate was used to overcome these deficiencies. DE-Appl. 101 14 169,6.
The hydrogels of the present invention overcome the deficiencies of the prior art by using a robust porous polymer- and solids-containing material, that is air-permeable, water-absorbing, and an optimal source of nutrients for plants.